In the conventional technology, artificial leather has been produced in a wet process wherein a textile product is impregnated and coated with a polyurethane resin solution in an organic solvent such as dimethylformamide (DMF) containing various additives including pigments, and then the organic solvent is extracted with water to form porous structure; or in a dry process wherein porous structure is formed by utilizing the volatility of an organic solvent.
In those production processes, a lot of organic solvents used in the processes have been causing problems in work place and of environmental pollution. In addition, those organic solvents must have been removed completely from resultant artificial leather. Incomplete elimination of organic solvents gives adverse effects on workers in production processes and users or wearers of products. And a lot of energy, labor and expenses have been required for preventing water and air pollution and adverse effects on workers and wearers.
On the contrary, the solvent-free process with urethane resin emulsion require no recovering of organic solvents and improves working environment causing no water and air pollution. And the adverse effects on workers, users and wearers due to remained organic solvents in products can be remarkably minimized.
The conventional processes with urethane resin emulsion, in which pores are formed with foaming, can only produce porous structure consisting of separated air bubbles.
Such porous structure is not suitable for artificial leather. The porous structure that can only be produced with foaming has limited the end use and application field of the artificial leather produced with urethane resin emulsion.
A production process for fiber sheet containing uniform microporous structure of urethane resin and having soft handle, in which an aqueous resin composition consisting of urethane resin having carboxyl groups in its molecule, an inorganic salt, and a nonionic surfactant having a clouding point is employed for attaining the structure and handle, has been disclosed in Japanese Patent Laid Open Hei 11-335975. In the process, the microporous structure is produced through producing crystalline structure with water-insoluble nuclei formed with cross-linking a proper quantity of the said carboxyl groups contained in the urethane resin molecules and the polyvalent metal ions in the said inorganic salt, such as aluminum chloride or calcium chloride. And the quantity of the carboxyl groups contained the urethane resin molecules, in other words, the acid value of the solid portion of the urethane resin, influences on the microporous structure and the stability of the aqueous resin composition. A sufficient quantity of carboxyl groups in the urethane resin molecules for producing satisfactory microporous structure deteriorates the stability of the aqueous resin composition. Thus urethane resins containing limited quantity of carboxyl groups must be applied for the process.
The present invention provides a process for producing porous structure, consisting of continuous pores and suitable for artificial leather, with a specific aqueous emulsion of urethane resin for the purpose of eliminating the above-mentioned environmental pollution in the production process with a urethane resin solution in organic solvent and adverse effect on human health caused from remained organic solvent. In addition, the present invention provides porous structure that solves the problems caused from nonuniform pores produced with conventional aqueous urethane resin emulsion and has a property similar to the porous structure produced with conventional urethane resin solution in organic solvent.